High-temperature combustion flue gas discharged from a thermal power plant, a gas turbine or the like contains nitrogen oxide, and to discharge the flue gas, nitrogen oxide in flue gas needs to be removed. Therefore, NOx removal equipment is installed on a downstream side of a combustion engine, and a reducing agent is injected from an injection nozzle into combustion flue gas so that the reducing agent reductively reacts with nitrogen oxide (NO, NO2), thereby decomposing combustion flue gas into harmless nitrogen (N2) and water (H2O). At the time of this operation, in a method of removing nitrogen oxide from flue gas by using a NOx removal catalyst, ammonia (NH3), urea or the like is normally added thereto due to the necessity of causing a sufficient denitration reaction.
Conventionally, when nitrogen oxide in flue gas is decreased, ammonia is added thereto as a reducing agent, and when a NOx removal catalyst is used, processing is performed normally in a high-temperature region of 300° C. or higher. The denitration reaction makes progress according to following equation (1), and nitrogen oxide is decomposed into N2 and H2O in a reaction between 1 mole of NO and 1 mole of NH3.4NH3+4NO+O2→4N2+6H2O  (1)
However, in the conventionally used catalyst, when a temperature becomes as high as 450° C. or higher, an oxidation reaction of NH3 itself makes progress according to following equation (2) or equation (3), other than the above equation (1).2NH3+ 5/2O2→2NO+3H2O  (2)4NH3+3O2→2N2+6H2O  (3)
Due to the reaction according to the equation (2) or the equation (3), NH3 is not effectively used for reduction of NO, and the NOx removal performance decreases with an increase of the temperature. Therefore, for example, when gas turbine outlet flue gas of 500° C. or higher is to be processed, the reducing agent is not sufficient, and thus application of a conventional reduction process accompanying ammonia addition has been difficult.
Meanwhile, as a removal method of nitrogen oxide at a high temperature, a technique of using titanium oxide containing heat-resistant inorganic fiber as a carrier has been reported (see Patent Literature 1).